1. Field of the Invention
This invention relates to a hetero-bipolar transistor (HBT), particularly to an HBT made of group III-V compound semiconductor materials.
2. Related Prior Art
A tunneling emitter bipolar transistor (TEBT) is a device that achieves a superior high frequency performance and a current gain. Such characteristics of the TEBT are reported in the IEEE Electron Device Letters, vol. EDL-7 pp. 416, (1986) and Applied Physics Letters, vol. 50(26) pp. 1915 (1987). The former document discloses the structure of the TEBT in which a base and an emitter are made of GaAs and a tunnel emitter of AlGaAs is inserted therebetween with a thickness of 3 nm to 20 nm. In this configuration, electrons can tunnel through the AlGaAs emitter layer, while the hole injection from the base layer to the emitter layer is suppressed because the tunnel probability for the hole is smaller as compared with electrons therethrough. Moreover, even in increasing the hole concentration in the base layer to decrease the base resistance, the hole injection to the emitter layer can be kept low. The TEBT achieves both an enhanced current gain and high frequency performance.
However, the AlGaAs tunnel emitter layer with a greater energy band gap as compared with the base layer must be sufficiently so as to allow electrons to tunnel therethrough. In addition, in order to attain such superior characteristics, the base layer must be heavily doped to reduce the base resistance. Such layer structure increases the carrier recombination between electrons in the emitter contact layer and holes in the base layer. This causes an increase of the base current, and accordingly decreases the current gain of the transistor.